Display systems (whether projection-based or direct-view) that use field sequential color techniques to generate color are known to exhibit highly undesirable visual artifacts easily perceived by the observer under certain circumstances. Field sequential color displays emit (for example) the red, green, and blue components of an image sequentially, rather than simultaneously, tied to a rapid refresh cycling time. If the frame rate is sufficiently high, and the observer's eyes are not moving relative to the screen (due to target tracking or other head/eye movement), the results are satisfactory and indistinguishable from video output generated by more conventional techniques (viz., that segregate colors spatially using red, green, and blue sub-pixels, rather than temporally as is done with field sequential color techniques).
However, in many display applications the observer's eye does partake of motion relative to the display screen (rotational motions of the eye in its socket, saccadic motions, translational head motions, etc.), such motions usually being correlated with target tracking (following an image on the display as it moves across the display surface). In the case of such image tracking, which involves oculomotor-driven rotation of the eye in its socket as the observer follows an object moving on the display screen, the object's component primary colors (red, green, and blue, for example) arrive at the observer's retina at different times. Even at a high frame rate of 60 frames per second, the red, green, and blue information from the display arrives at the retina 5.5 milliseconds apart. If the retina is in rotational motion, as would be the case if the observer were tracking an image (hereafter “target”) that was moving across the display, the red, green, and blue information comprising the target would hit the retina at different places. A target that is gray in actual color will split into its separate red, green, and blue components distributed in overlap fashion along the path of retinal rotation. The faster the eye moves, the more severe the “image breakup,” the decomposition of the individual colors comprising the target due to where those primary components strike the observer's retina. These visual artifacts have proven to be a barrier to the adoption of field sequential color displays in many critical applications, including video systems for training fighter pilots using flight simulation. A trainee in such a flight simulator needs to encounter an environment that matches reality closely, and a discontinuous smear of red, green, and blue ghost images that are not overlapped properly do not constitute an acceptably simulated target when the trainee is expecting to see the grey winged fuselage of an enemy fighter plane in the crosshairs.
The display system disclosed in U.S. Pat. No. 5,319,491, which is incorporated by reference in its entirety herein, as representative of a larger class of direct view field sequential color-based devices, illustrates the fundamental principles at play within such devices. Such a device is able to selectively frustrate the light undergoing total internal reflection within a (generally) planar waveguide. When such frustration occurs, the region of frustration constitutes a pixel suited to external control. Such pixels can be configured as a MEMS device, and more specifically as a parallel plate capacitor system that propels a deformable membrane between two different positions and/or shapes, one corresponding to a quiescent, inactive state where frustrated total internal reflection (FTIR) does not occur due to inadequate proximity of the membrane to the waveguide, and an active, coupled state where FTIR does occur due to adequate proximity, said two states corresponding to an off and on state for the pixel. A rectangular array of such MEMS-based pixel regions, which are often controlled by electrical/electronic means, is fabricated upon the top active surface of the planar waveguide. This aggregate MEMS-based structure, when suitably configured, functions as a video display capable of color generation by exploiting field sequential color and pulse width modulation techniques. Red, green, and blue light are sequentially inserted into the edge of the planar waveguide, and the pixels are opened or closed (activated or deactivated) appropriately, such that the duration of a pixel's being opened (activated) determines how much light is emitted from it, gray scale being determined by pulse width modulation.
Other direct view displays may use field sequential color techniques, but substitute amplitude modulation for pulse width modulation. For example, a monochromatic liquid crystal display with suitably fast switching times can be turned into a field sequential color display by replacing the white back light with a back light that can sequentially emit red, green, and blue light in sufficiently rapid succession. Liquid crystal pixels are variable opacity windows that modulate the amount of light passing through them by amplitude modulation rather than pulse width modulation. Undesirable visual artifacts arise for these systems as well, and for the same reason: the respective primary components of the image (target) fall on a moving retina at different places, causing the apparent breakup of the target as perceived.
Projection-based systems can also use field sequential color. The DLP (digital light processor) developed by Texas Instruments, Inc., employs a dense array of deformable micro-mirror structures that are used to create an image when red, green, and blue lights are directed onto them in rapid consecutive sequence. Light from activated micromirror pixels passes through a lens system and is focused on the final projection screen for viewing, while light striking inactive pixels are not sent through the lens system. Such systems tend to use pulse width modulation to generate gray scale. The red, green, and blue light being directed onto the micromirror array can be created either directly (with discrete red, green, and blue sources) or as the result of white light passing through a rotating color wheel composed of red, green, and blue filter segments. In either case, the undesirable artifacts are clearly visible on the image projected onto the display screen, for the same reason they appear in a direct view device: the respective red, green, and blue images do not fall on the moving retina at the same place, causing spatial decomposition and the resulting color breakup artifact.
Field sequential color displays bring many advantages to the display sector, whether one considers direct view displays (such as flat panel display systems) or projection-based systems. For example, in a flat panel display that uses conventional spatially-modulated color with red, green, and blue sub-pixels comprising an individual pixel, three control elements (usually thin film transistors) are required to separately control the red, green, and blue intensities from the pixel. A display with one million pixels would require three million transistors to drive it in color. The corresponding display using temporally-modulated color (field sequential color) needs only one thin film transistor per pixel, reducing the amount of transistors distributed over the display surface from three million to one million—an improvement that has significant implications for yield and production cost. Moreover, a field sequential color pixel can be much larger, since it fits in the area that would normally be occupied by three sub-pixels (red, green and blue), further improving production yield and reducing aperture drain (surface area on a display not given over to light emission). Conversely, this geometric advantage can be exploited to improve pixel densities without the heavy control overhead associated with standard sub-pixel-based architectures, yielding superior resolutions without exponential price increases. Accordingly, field sequential color displays have much to recommend them. But their utility in applications where color image breakup is unacceptable is sharply curtailed.
Therefore, there is a need in the art for a means to mitigate and suppress the color image breakup artifacts traditionally associated with displays that employ the principle of field sequential color generation, whether in a direct view or a projection-based system. A display device that enjoys the benefits of field sequential color operation without generating unacceptable motion artifacts would bring the benefits of field sequential architectures (direct view and projection-based) to bear on applications where those benefits are most needed, e.g., critical flight simulation display systems.